1. Technical Field
This invention is directed toward a system and method for video compression/decompression. More specifically, this invention is directed towards the generation, coding and transmission of a video form.
2. Background Art
Wireless networks have been deployed rapidly in recent years. GPRS (General Packet Radio Service) and CDMA 1X (Code Division Multiple Access) as 2.5 G solutions to wide area wireless networks are available in increasingly more regions in Europe, North America and Southeast Asia. Wireless LAN 802.11 and Bluetooth also compete strongly for local area wireless networks. The fast expansion of wireless networks calls for rich content and services for consumers. However, due to limited channel bandwidths in these wireless networks and the weak processing power of mobile devices, conventional media contents are difficult to distribute.
Bi-level video [1] is an effective solution for low bandwidth mobile video conferencing, where previously there did not exist suitable video coding technology for current wireless network and mobile device conditions. It was observed that although conventional video processing and coding technologies such as MPEG1/2/4 [2] and H.261/263 [3, 4] could also code video for low bit rates, the resultant images usually looked like a collection of color blocks and the motion in the scene became discontinuous. The block artifacts of these methods originate from the common architecture of MPEG1/2/4 and H.261/263, i.e. discrete cosine transform (DCT)-based coding. In DCT-based coding, low spatial frequency values that represent the “basic colors” of the blocks possess high priority. However, in video communications, facial expressions that are represented by the motions of the outlines of the face, eyes, eyebrows and mouth deliver more information than the basic colors of the face. Bi-level video uses bi-level images to represent these facial expressions, which results in very high compression ratios. Experiments show that at low bandwidths, bi-level video provides clearer shape, smoother motion, shorter initial latency and much cheaper computational cost than do DCT-based technologies. Bi-level video is especially suitable for small mobile devices such as handheld personal computers (PCs), palm-size PCs and mobile phones that possess small display screens and light computational power, and that work in wireless networks with limited bandwidths.
In bi-level video, scenes are always represented by two colors, usually black and white. Although black and white are sufficient to describe the outlines of a scene, the visual quality is not very satisfactory. Given that many mobile devices are now able to display at least four levels of grayscale, users of a research prototype for mobile video conferencing have expressed a desire for improved video that contains more gray values and has better visual quality. With this improved video, bit rates must also be kept low.
After reviewing existing video technologies that cover different bandwidth ranges, it was found that MPEG/H.26x performs well in the bandwidth range greater than about 40 Kbps and bi-level video works well in the range of 10-20 Kbps for quarter common intermediate format (QCIF) size (e.g., 144 lines and 176 pixels per line). However, the visual quality of bi-level video can no longer be improved even if greater bandwidth is assigned to it. The task is then how to improve the visual quality of bi-level video in the bandwidth range of 20-40 Kbps or how to design a new video form that can work well in this range. It is very important to develop a video form to fit into the 20-40 Kbps bandwidth range because this is the range that 2.5 G wireless networks such as GPRS and CDMA 1X can stably provide, although the theoretical bandwidths of GPRS and CDMA 1X are 115 Kbps and 153.6 Kbps, respectively.
It is noted that in the preceding paragraphs, as well as in the remainder of this specification, the description refers to various individual publications identified by a numeric designator contained within a pair of brackets. For example, such a reference may be identified by reciting, “reference [1]” or simply “[1]”. A listing of the publications corresponding to each designator can be found at the end of the Detailed Description section.